Thermostats for heating furnaces and/or air cooling systems (hereinafter collectively referred to as "furnaces") of the type employed in residences and many commercial and industrial buildings generally include storage means for a desired temperature set-point, means for measuring the actual temperature within the building and means for switching the furnace on or off as a function of differences between the set-point temperature and the actual temperature.
To prevent the furnace from being rapidly turned on and then off as it hunts about the set-point temperature, these thermostats have a built-in dead zone; i.e., the temperature at which the thermostat contacts are closed to energize the furnace is slightly below the temperature at which they open after the furnace has warmed the room containing the thermostat. For example, when the set-point of the thermostat is adjusted to 70.degree. F., the furnace burner may be energized when the temperature drops to 69.degree. and de-energized when the room is heated to 71.degree.. The temperature differential of the dead zone is determined on the basis of several considerations: if the dead zone is made very small the furnace will turn on and off relatively rapidly when the room temperature is close to the set-point, causing increased wear on the valves and the like, a decreased thermal efficiency because of the energy required to repeatedly heat the furnace and cooling ducts, and an annoying noise level produced by the rapid changes in air movement. A larger dead zone produces a higher thermal efficiency and less system wear and noise but the occupants will notice temperature changes in excess of about 3.degree., imposing a practical limit on the maximum width of the dead zone.
In practice, the dead zone is typically set to something slightly less than 3.degree. because after the thermostat contacts are opened, turning off the burner, the furnace continues to raise the temperature of the house for a short period of time while the blower forces previously heated air into the room. This produces a "thermal overshoot" in which the temperature of the house reaches a maximum some period of time after the furnace burner is de-energized and the maximum temperature excursion into the thermostat room is somewhat higher than the temperature differential represented by the dead zone in the thermostat. The extent to which this thermal overshoot occurs varies as a function of thermal loss of the heated building to the exterior; in the winter it may be very slight and in the spring when a relatively small temperature differential exists between the heated building and the exterior, it will be substantially larger because the heated air left in the furnace and ducts when the burner is extinguished will produce a larger temperature rise in the house. The thermal overshoot will also vary as the function of the building construction. A building with masonary walls must be heated for a longer period of time than a wood frame building to produce a specific temperature change. The dead zone setting is necessarily a compromise which produces an overly large temperature excursion in warm weather and an unnecessarily short excursion in cold weather.
A similar compromise must be made in setting the lower temperature limit of the dead zone. The room temperature continues to drop for some period of time after the conventional thermostat recognizes the low temperature setting of the dead zone and de-energizes the furnace burner. This lag results from the time required for the furnace bonnet to heat up to a sufficient temperature to allow the blower to be started. In cold, windy weather this "undershoot" will be larger than in warm still weather when there is a low rate of heat loss from the house.
These compromises affect the thermal efficiency of the heating system. If the burner's cycle time for a specific building could be optimally adjusted for each set of atmospheric conditions the thermal efficiency of the furnace system would be improved.